I can't tell if this is behavioral or an illness. Any insight? He seems to stop when I give him a neck massage, and I haven't noticed him doing it in his sleep. I am trying to figure out of I can hold off until our Friday vet appointment or if I need to try to get in tomorrow.

It does sound like hissing to me as well. To my understanding if they make that sound while going to the potty it could mean possible UTI while peeing or constipation while pooping. My gliders have made this sound periodically, sometimes just hear one make the sound when inside their pouch, so I know they are not going to potty, they do not go inside their pouch (unless in carrier, sometimes they have no choice). I have on more rare occasions seen one of my gliders make this noise while pooping, it doesn't last long (meaning not more than a day/night). Otherwise, I believe it's just another sound they make as form of communication. I am no expert though on glider sounds, just have read various reasons they make this sound.

If it is frequent and continuous I recommend a well check visit with his vet to be safe.

When I posted that, he had gone 2 days without eating. I stayed up until 4 a.m. to watch him, and he was definitely very constipated and seemed to be in pain. The sound was definitely excessive and out of the norm for him, which is why I was so concerned.

I took him to the vet the next morning (technically the same morning), and he had some diarrhea on the way there. Other than that, the vet said he looked great and seemed healthy. She did give him an antibiotic just to be safe, as well as something for his tummy. She said it was probably due to the stress of meeting his new cage mate. She didn't even care about the sound because she said it could mean too many conflicting things. He is eating more now and doing much better. Plus, they neutered my other guy a day early so I could avoid making two trips. Poor guy didn't get a last hurrah. lol

That is good that you got him right into the vet. It could have been a UTI or constipation.

Very happy to see that you are in tune to your little guy.